Many wireless communication systems require full duplex operation, in which a radio transceiver receives and transmits signals simultaneously. Because the transmitter typically outputs signals at a power level much higher than that of received signals, the receiver section of the transceiver often suffers self-interference from the transmitter. This self-interference, sometimes called crosstalk, self-jamming, or transmitter leakage, results from coupling of the transmitted signal to the receiver through any of several mechanisms. For instance, duplexer filters, designed to allow the receiver and transmitter to share an antenna, provide only limited isolation between the transmitter and receiver paths. Larger duplexer devices typically provide more isolation, but market pressures increasingly demand that wireless transceivers be very small. Transmitter leakage may also be coupled from one conductive path in a wireless transceiver to another, even where the two paths are not directly connected, or may result from crosstalk leaking through integrated circuit substrates, device packages, or printed circuit boards.
Transmitter leakage into the receiver path is typically down-converted, along with the received signal of interest. If a receiver comprised perfectly linear components, the down-converted transmitter leakage would often not be a problem, since the transmitter leakage signals would remain separated from the received signal of interest by the frequency duplex spacing, thus permitting effective filtering. However, in practical receivers, utilizing active components having limited linearity, transmitter leakage can result in interference, e.g., degraded signal-to-noise ratio, and thus can interfere with demodulation of the received signal. Transmitter leakage may desensitize a receiver, or, if large enough, even block the receiver from detecting received signals at all.
Recently, direct down-conversion radio receivers have become more widely used. These receivers are typically easy to integrate, thus reducing the total number of components as well as the cost. However, very high linearity in the receiver front-end is typically required. In particular, it can be quite difficult to meet system performance requirements in a direct down-conversion, frequency-duplexing receiver, such as requirements on the second-order intercept point (IIP2) caused by the transmitter leakage and the third-order intercept point (IIP3) caused by the interaction between transmitter leakage and strong interfering signals at half of the duplex distance from the desired signal.
Furthermore, as will be appreciated by those skilled in the art, continuing advances in integrated circuit technology, e.g., advanced CMOS technology, is driving reductions in feature size and corresponding reductions in supply voltages. Reductions in supply voltage result in less voltage headroom, so that device linearity requirements are even harder to meet. Thus, market pressures to reduce transceiver size, cost, and power consumption are presenting continuous challenges for radio receiver design.
Various solutions have been proposed to solve the interference problems caused by transmitter leakage. For instance, several approaches have been proposed that employ least-mean-square (LMS) adaptive filter techniques to cancel the transmitter leakage signal. In some of these approaches, the “error” caused by the transmitter leakage is measured and corrected at baseband (see, for example, U.S. Patent Application Publication 2007/0184782), while in others, the filtering is performed at radio frequencies (RF) (see, for example, U.S. Pat. No. 5,444,864). However, these approaches suffer from several limitations. First, because the residual error is related to the loop gain, larger loop gains result in lower cancellation error. However, large loop gains may cause stability problems. Another problem arises from the DC offset associated with the integrators used in LMS approaches; this problem can be difficult to solve. A third problem relates to convergence, as there may be one or more initial conditions from which the LMS algorithm is unable to achieve a convergent result.
Thus, improved solutions for reducing interference caused by transmit leakage in frequency-duplexing transceivers are needed. In particular, solutions suitable for use in portable, direct down-conversion RF transceivers, such as those used in mobile telephone or other duplex wireless communication systems, are needed.